SparkIgnition

Spark ignition refers to the method of initiating combustion in internal combustion engines by an electric spark. In spark-ignition (SI) engines, the air–fuel mixture is ignited by a spark plug fired by an ignition system. SI engines are typically gasoline engines but can burn other fuels such as natural gas or ethanol blends. They are usually contrasted with compression-ignition (diesel) engines, where ignition occurs due to high compression rather than a spark.

Operation and control: The intake mixture is created by fuel delivery systems such as port fuel injection or gasoline direct injection. The piston compresses the mixture, and near top dead center the ignition system fires the spark plug, producing a flame kernel that rapidly propagates through the mixture and drives the power stroke. Spark timing, or spark advance, is adjusted based on engine speed, load, and temperature to maximize efficiency and power while avoiding knock. Modern SI engines rely on electronic control units and sensors to optimize timing and to detect misfires or abnormal combustion with knock sensors.

Fuel, efficiency, and design: The ideal air–fuel ratio for many SI gasoline engines is around the stoichiometric value of about 14.7:1, though lean or rich mixtures are used in different operating modes. Fuel quality, including octane rating, affects resistance to knock. SI engines accommodate a range of fuels, including gasoline with ethanol blends. Technological advances such as electronic ignition, coil-on-plug systems, and sometimes turbocharging or direct injection have improved efficiency, power, and emission control compared with early ignition systems.

Applications and issues: SI engines remain common in passenger vehicles and light aircraft, often integrated with emissions control systems and hybrid powertrains. Common issues include spark plug fouling, misfires, and knock, which are managed through sensor feedback, control strategies, and maintenance.